1. Technical Field
The present invention relates to discharge lamp driving devices, projectors, and methods for driving a discharge lamp.
2. Related Art
In the past, a projector that projects a picture onto a screen has had the technique of controlling the color balance or the brightness by driving a discharge lamp used in a light source of the projector by performing amplitude modulation on a drive current (hereinafter also referred to as a lamp current) of the discharge lamp. For example, there is a projector that performs time division on a white light emitted from the light source by using a color wheel including R (red), G (green), and B (blue) color filters, modulates the lights obtained by the time division into picture signals of these colors, and projects a picture. In such a projector, a method by which the color balance is controlled by driving the discharge lamp so that the lamp current thereof is increased during the interval that a light is passing through a predetermined color filter is proposed (for example, JP-A-2007-121971).
Moreover, projectors that project a three-dimensional (hereinafter also referred to as 3D) picture include a time-division 3D projector that alternately projects a picture for a right eye and a picture for a left eye by time division. The time-division 3D projector has shutter glasses that alternately open and close a liquid crystal shutter for a left eye and a liquid crystal shutter for a right eye in synchronization with the picture for a left eye and the picture for a right eye which are projected. The user can view 3D pictures by viewing the projected pictures through the shutter glasses. In this case, as a result of a picture in one frame being divided into two pictures: a picture for a left eye and a picture for a right eye by time division, the brightness of the picture for a left eye which is projected and the brightness of the picture for a right eye which is projected are reduced to half, and the picture that reaches the left eye and the picture that reaches the right eye become dark. Furthermore, when switching between the picture for a left eye and the picture for a right eye is performed, since a period in which the switching is performed includes a period in which control to close the liquid crystal shutter for a left eye and the liquid crystal shutter for a right eye at the same time is performed, the pictures that reach the eyes become even darker. To solve this problem, there is a method by which amplitude modulation is performed on the lamp current in such a way that the lamp current is increased in a period in which one of the liquid crystal shutter for a left eye and the liquid crystal shutter for a right eye is opened and the lamp current is decreased in a period in which both of the liquid crystal shutters are closed. By this method, it is possible to make the pictures that reach the eyes bright without increasing the average power of drive power (hereinafter also referred to as lamp power) of the discharge lamp.
However, since a drive voltage (hereinafter also referred to as a lamp voltage) of the discharge lamp has initial variations and a temporal change, even when the discharge lamp is driven in such a way that the average power which is supplied to the discharge lamp becomes constant, the lamp current increases when the lamp voltage decreases. Therefore, when the discharge lamp driving device performs amplitude modulation on the lamp current of the discharge lamp, even when the average current of the lamp current is less than or equal to the rated current, the lamp current may exceed the rated current in a period in which the lamp current is increased. In this case, heat generation in the discharge lamp driving device becomes a problem, and solving this problem undesirably necessitates an increase in the cost of the parts used in the discharge lamp driving device or an increase in the sizes of the parts.